The present invention relates to a subcarrier signal generator for a stereo tuner, and more particularly to a device for generating a sine-wave subcarrier signal in synchronism with a stereo pilot signal in an FM stereo tuner.
Multiplex demodulators in FM stereo tuners generate a 38-KHz sine-wave subcarrier signal and multiply this signal with a stereo composite signal to demodulate a subchannel signal in the stereo composite signal. There has previously been used a PLL (phase-locked loop) circuit as shown in FIG. 1 of the accompanying drawings to produce the 38-KHz subcarrier signal in synchronism with a pilot signal.
The pilot signal a is compared by a phase comparator 1 with a 19-KHz signal f from a 1/2 (one-half) frequency divider 2, and the output from the phase comparator 1 is supplied through a low-pass filter 3 and DC amplifier 4 to a voltage-controlled oscillator 5. The voltage-controlled oscillator 5 generates trigger pulses b of 76-KHz, which are converted by a 1/2 frequency divider 6 into a 38-KHz rectangular wave c, which is then fed to a bandpass filer 7 (which may be a low-pass filter) which produces a 38-KHz sine-wave subcarrier signal d. The subcarrier signal d is converted by a level comparator 8 into an in-phase rectangular wave signal e, which is then converted by the frequency divider 2 into the 19-KHz signal f. The sine-wave subcarrier signal d is thus produced in synchronism with the stereo pilot signal a in the stereo composite signal. FIGS. 2(a) through (f) respectively show the waveforms of the signals a through f generated in the circuit of FIG. 1.
The 38-KHz rectangular wave signal supplied to the bandpass filter 7 can be expressed by: EQU V(t)=(4/.pi.) sin .omega.st+(4/3.pi.) sin 3.omega.st+(4/5.pi.) sin 4.omega.st+ (1)
where .omega.s is the angular frequency of the subcarrier signal. The waveform defined by the equation (1) is as shown in FIG. 3(a), and its frequency spectrum is as illustrated in FIG. 3(b). Since the 38-KHz rectangular wave signal contains odd-numbered harmonics such as the third, fifth and so on, having levels of 1/3, 1/5 etc, respectively, of the fundamental, the bandpass filter 7 is required to have transmission characteristics as shown in FIG. 4. This is disadvantageous in that the bandpass filter 7 requires many filter elements, and the 38-KHz sine wave tends to vary in level due to temperature drifts as the gain starts to drop in the vicinity of 38 KHz.